This invention relates to a coin checking device for use in a coin handling machine for discriminating genuine coins from counterfeit.
Conventionally, the discrimination of genuine coins from counterfeit has been effected by the detection of coin diameter, thickness, weight, material, etc. The following method has been used for discriminating the material properties of the coins: A pair of coils are mounted opposite to each other so that the coin path is located between the coils. An a.c. voltage is applied to one of the coils and when the coin passes between the coils, variation of voltage induced in the other coil is detected, and this variation value is compared against a predetermined reference value to determine what material the coin is made of.
In the above-mentioned method, the reference value is maintained to remain at a constant level. In such a case, discrimination errors are generated, by fluctuation of detection threshold level caused by variation of temperature, or drift arising in oscillators employed for applying such a.c. voltage to one of the coils or in amplifiers employed for amplifying such induced voltage. More specifically, when coins to be discriminated are of a very similar material (for example a 50 yen coin and a 100 yen coin, or a 500 yen coin and a (Korean) 500 won coin), the method is disadvantageous in that the differences in the said variation in induced voltage produced by the material of the coins are so slight that such level fluctuation caused by drift or variation of temperature gives rise to erroneous discrimination. Although in order to avoid such erroneous discrimination, there has been proposed means for suppressing level threshold fluctuation, this involves the use of a constant temperature bath or highly expensive low-drift elements.